90% renewables

 Study after study has shown that we can get to about 90% renewables if we have a grid with mixed wind and solar and about 4 hours of storage.  In the chart below, I have taken Lazard's LCOE calculations and BNEF's battery-pack costs and estimated what achieving 90% renewables would cost, assuming 30% wind/solar overcapacity and 4 hours of li-ion storage.  The additional 10% could come from a variety of sources: hydro, small hydro, wave power, tidal power, biomass, SMRs, green gas, or peaking gas, but these are not costed.   Some of them are more expensive than this model, so overall costs for the whole grid will be higher than shown.

Assumptions:

• Battery-packs are co-located with wind/solar farms with shared grid connections and inverters
• Excess output from wind/solar farms is not curtailed (variability doesn't just mean there's too little wind/sunshine, but also sometimes too much) but used instead to produce green hydrogen.
• The power used to charge the batteries is fully paid far, and isn't just the generation surplus, diverted into storage to prevent curtailment.  I also assumed it would earn nothing when sold, which, now that I think about it is unrealistic.  But conservative. 
• Batteries are not managed to "ride the cycle", i.e, they don't take advantage of movements in the wholesale price to make money, by charging when prices are low and discharging when they are high.
• The projections are derived from extensions of the average rates of decline for the last 5 years, except coal costs (new-build and marginal) are assumed to be flat.
• The cost of finance is assumed to equal the inflation rate in retail electricity prices.  However, this may be invalid, as the expansion in renewables will cut the costs of electricity.

What the chart shows is that  the cost of a wind+solar grid, with 30% overcapacity, and 4 hours of storage became cheaper than the cost of new-build coal in 2015 and will fall below the marginal cost of coal in 2025.   This means that after 2025, there will be rapid closures of existing coal power stations, even if they are not fully depreciated and paid off because the cost of brand-new wind and solar farms, with excess capacity and storage to "firm" output, will be cheaper than digging up the coal, shipping it and burning it.